I've just finished two of the MFOS SVF modules and they both have the same issue / problem.

It appears to be in the Resonance part of the circuit though it's strange that they both have the same issue. I've gone through the board several times and all components are correct. The only changes are small value differences in resistors. Where Ray has 49k9 resistors I have 52k substitutes.

I've attached a MP3 of the audio output which starts with the resonance in the middle (or there abouts) which emulates the resonance pot not being connected. Then I change the frequency cut off which responds as you'd expect but then I turn up the res and it does the weird stuff. The level this puts out is quite high and I needed a pot on the output to stop it clipping my mixer. Sorry no real voltage output measured.

Any thoughts or leads would be appreciated.

PS - If you're interested here's the total synth I am currently building - It's been titled the MFOS - Sound Box Effets 2 - not sure why I went for french naming ? Just did. - http://amongstprojects.blogspot.com.au/

To be more constructive, I'm not entirely sure what I'm hearing there.

Something like starting with a known waveform at a fixed mid-range frequency and then adjusting the filter would be more clear about the problem. What I hear in the MP3 sounds like it's very low frequency to start with.

Ok. The audio source into the filter is white noise off the Experimenters board. Firstly I adjust the resonance control which introduces the low frequency osc sound. Then I change the cut off frequency which shows that this function is still working. Then I increase the resonance which sounds like I am applying a square wave lfo. When this sound appears the audio signal level goes off the scale. I have no real way of measuring this level but it is quite high - several volts above the original audio level.

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